This application claims the benefit, under 35 U.S.C. § 365 of French Patent Application 04300426.6, filed Jul. 5, 2004.
The present invention relates to interfaces for transmitting and receiving a HF modulated video signal of high bandwidth, in particular for a video camera and a base station with which the video camera communicates.
Professional high definition television (HDTV) broadcast cameras for studio and electronic field productions today use either an optical fibre or a special coaxial cable, referred to as a triax cable, for communicating with a base station where image data from one or more cameras are collected and processed. Optical fibres allow very high data rates between the camera and the base station, so that many experts expect that in the long run optical fibre technology will become standard for connecting cameras and base stations. At present, however, fibre technology has some difficulties in penetrating the market, because in many facilities where professional video cameras are used, for example in sport stadiums, triax cables are installed, whereas optical fibres are not. Accordingly, a producer who uses triax transmission technology, for recording an event, can bring his cameras to a given location, install his base station, and can interconnect cameras and base stations using triax cables that are present on the location. A producer who uses optical fibre technology cannot rely on such fibres being present, so that for recording the same event, he must not only install his cameras and base station but must also do the wiring. This does not only increase the cost of setting up his equipment, it also decreases its reliability. In addition, current technology of fibre optic connectors are more sensitive to contaminations by dust and humidity compared to triax connectors. Therefore, triax technology will probably be continued to be used for quite a long time in the future.
In a triax cable, information is transferred from the camera to the base station and vice versa using a plurality of carriers at different frequencies. Some of these have components of the camera's video signal modulated onto them in order to transfer the video signal to the base station, other carriers are modulated in the base station in order to transfer control information from there to the camera. All these carriers suffer an attenuation in the cable that imposes an upper limit for the length the triax cable can have. For standard definition cameras, this upper limit is at approx. 3 km; for HDTV cameras, in which the bandwidth of the video signal is higher, the usable maximum length of the triax cable reduces to about 1500 m.
In a number of applications, especially when recording a sports event, it is interesting to generate more pictures per second than under normal operating conditions. The extra pictures generated are used to play back certain actions in slow motion while maintaining sharpness in moving objects. This is a common feature in standard definition TV camera systems. It would be desirable to provide the same feature in HDTV cameras, too, but the problem arises that if the extra pictures also have to be transmitted via the existing triax cables, more bandwidth is needed. I.e. whereas at present a HDTV video signal occupies a frequency range between approx. 25 and 130 MHz, the spectrum of a HDTV Signal with an increased image rate will have to extend to still higher frequencies. However, since attenuation increases with frequency, this causes the maximum useable length of the cable to decrease still further. If the maximum useable length becomes less than the length of the installed cable, it is quite evident that the system will become inoperable.